1. Field of the Invention
The present invention relates to a method for wireless data transmission between a base station and one or more transponders.
2. Description of the Background Art
Such transmission methods between one or more base stations or readers and one or more transponders may be used for example in contactless identification systems or so-called radio frequency identification (RFID) systems. Sensors, for example for temperature measurement, may also be integrated on the transponder. Such transponders may also be referred to as remote sensors.
The transponders or the transmitting and receiving devices thereof are not usually provided with an active transmitter for the data transmission to the base station. Such non-active systems are referred to as passive systems if they do not have their own energy supply, and are referred to as semipassive systems if they have their own energy supply. Passive transponders draw the energy required to supply them from the electromagnetic field emitted by the base station.
So-called backscatter coupling is generally used for data transmission from a transponder to the base station with UHF or microwaves in the far field of the base. For this purpose, the base station emits electromagnetic carrier waves which are modulated and reflected by the transmitting and receiving device of the transponder in accordance with the data to be transmitted to the base station by means of a modulation method. The typical modulation methods for this purpose include amplitude modulation, phase modulation, and amplitude shift keying (ASK) subcarrier modulation, in which a frequency or a phase angle of the subcarrier is changed.
The data to be transmitted comprise successive characters which are usually coded and transmitted by a transponder in each case within a predefined time or character interval. In the case of a binary transmission, the value of a character is either “0” or “1”. Various methods are known for coding a character.
One coding method, typically called 3phase1 coding, is described, for example, in ISO WD 18000-6 mode 3 dated Jan. 2, 2002. In this case, the significance of a character is coded by the time difference between a synchronization mark generated by the base station and a state change of a modulation signal that is generated by a transponder. The coding is effected synchronously with the synchronization marks. The synchronization marks are also referred to as “notch”.
Another coding method is typically called non return to zero inverted (NRZI) coding. In this case, a “1” is represented by an edge or modulation state change in a serially transmitted signal, known as a line code, at the beginning of a character or bit interval. If a “0” is transmitted, a change in the serial signal or in the modulation state does not take place. A variant of this method is typically called soft-NRZI coding, in which an edge change takes place in an offset manner with respect to a clock signal or a synchronization mark. Such a method is described in applicant's application, which has the German Application No. DE 10 2004 01 3885.0, which was filed on Mar. 16, 2004, and which is incorporated herein by reference.
Another coding method is typically called FM0 or differential biphase (DBP) coding, which is described for example in the textbook by Klaus Finkenzeller, RFID-Handbuch [RFID manual], 3rd edition, HANSER, 2002, see in particular chapter 6.1 Coding in baseband. In this case, a “0” is represented by an arbitrary edge in the center of the bit interval. If a “1” is transmitted, an edge change does not take place in the center of the interval. Independently of the value of the character to be transmitted, an edge change is generated at the start of each bit interval, thereby facilitating clock recovery.
Since the signals backscattered by the transponders have a very low power, corruption of the transmitted characters or data can easily occur on account of interference sources in the vicinity of the transponders. By way of example, the power of the backscattered signals may be in the region of −100 dBm whereas the power of interference signals, for example caused by mobile telephones, may be in the region of −54 dBm. In order to increase the transmission security, safeguard characters are usually sent subsequent to (payload) characters or (payload) data to be transmitted, which are formed from the data with the aid of mathematical operations. What are usually involved in this case are so-called parity bits and/or so-called cyclic redundancy codes having a specific length, generally 16 bits.
Attaching the safeguard characters increases the time required for the data transmission in accordance with the number of safeguard characters or safeguard bits additionally transmitted. This adversely affects the usable transmission bandwidth since the safeguard characters serve only for safeguarding and not for transmitting payload data.
One possibility for compensating for this increase in the transmission time is to increase the transmission rate. However, increasing the transmission rate presupposes increasing the oscillator frequency in the respective transponder, as a result of which its current consumption increases and the transmission range consequently decreases. Furthermore, the required transmission bandwidth increases. This is possible, however, only within the limits of the respective standards prescribed by the legislator. Since the noise energy likewise increases with the bandwidth, a higher sensitivity of the receiving part of the base station is additionally necessary.